At present, bonding wires, made of highly pure 4N-type gold (purity>99.99% by mass) and having a wire diameter of about 20 to about 50 μm, are used for connecting electrodes on a semiconductor element to external terminals. An ultrasonic wave-combined thermal press-adhering system has generally been used as technology for connecting the bonding wires necessitating a generally employed bonding device and a capillary jig used for inserting the wires therein for connection. An end of a wire is heated and melted by arc heat input to form a ball utilizing the surface tension, the ball is press-adhered onto the electrode of the semiconductor element heated in a range of 150 to 300° C. and, then, the wire is directly wedge-joined to the external lead side by ultrasonic wave press-adhesion. In order for the device to be used as a semiconductor device such as a transistor or an IC, the device is molded with an epoxy resin after the wires are bonded to protect the silicon chip, the bonding wires and the lead frame of a portion on where the silicon chip is mounted. In addition to improving their properties, increasing importance has been placed on improving their relationship to the surrounding members and on improving overall performance such as use and reliability.
Accompanying the trend toward highly densely integrating the semiconductor element and decreasing the thickness, the gold bonding wires have to satisfy diverse properties, such as elongating the gold bonding wires, decreasing the thickness of the wires, forming high loops or forming low loops for decreasing the thickness of the semiconductor element.
A material to which the bonding wires are to be joined, too, is changing. The wiring and electrode materials on the silicon substrate now use Cu and a Cu alloy which are suited to being highly densely integrated, in addition to using the traditional Al alloy. Even in the electrode members of Al alloy, Cu and Cu alloy, it has been urged to employ a small ball junction to meet narrow pitches, and it is becoming important to maintain junction strength, ball deformation and reliability in a high-temperature junction.
To meet the demands for highly densely integrating the semiconductor elements, strict requirements have been placed on narrowing the pitch, decreasing the size of the wires, increasing the number of pins, increasing the length of the wires, and obtaining a high degree of junction property in connecting the gold bonding wires.
In, for example, a resin-molding step where a highly viscous thermosetting epoxy resin is injected at high speed, a problem arises in that the wires are deformed to come in contact with the neighboring wires. Besides, while pitches are becoming narrow, and wires are becoming long and fine, it has been desired to suppress the deformation of wires (hereinafter also called wire sweep), by even a small amount, at the time of being molded with the resin. It has been strongly demanded to narrow the pitch. At present, mass production is at a level of maintaining a pitch of 60 μm. A pitch of 50 μm has also been developed, and it is expected that a very narrow pitch of 45 μm, which several years ago was considered to be a limit of the ball junction, may be put into practice within two to three years. In a road map related to the mounting technology, it is expected that a technology for realizing a pitch of 20 μm will be realized in the future.
Among many kinds of semiconductor packages, BGAs (ball grid arrays) and CSPs (chip size packages) have narrow pitches. Their mounting forms are based on the use of a board and a tape having fewer limitations, such as reducing the gap among the leads, than that of the conventional lead frame structures. When the board and tape are to be used, attention must be given so that the junction can be effected at a bonding temperature in a temperature region of as low as 150 to 170° C. This is a drop in temperature by several tens of degrees in centigrade as compared to the bonding effected at 210 to 300° C. in the case of the lead frame. It has therefore been desired to accomplish a narrow-pitch junction at a low temperature. Due to a delay in diffusion at low temperatures, due to a decrease in the junction area and due to very fine capillary ends, both the ball junction and the wedge junction have to satisfy very strict properties and reliability.
As basic properties of the wires satisfying such a requirement, it has been desired that overall properties are satisfied, such that the loop shape can be highly precisely controlled in the step of bonding, the junction is improved relative to the electrode portions and to the lead portions, and deformation of wires is suppressed in the mounting step after the bonding step.
In order to reinforce the bonding wires, so far, it was common to add a plurality of alloy elements. In the highly pure gold bonding wires which are now the main stream, addition of the alloy elements is limited to several ppm to several tens of ppm to prevent oxidation of the ball portions or a rise in the electric resistance. Therefore, though the loop controllability and junction properties are superior, it is not still satisfactory concerning suppressing the wire deformation or the strength of the heat-affected portions (neck portions) at the time of forming the balls. In recent years, high alloy wires for which the amount of addition is increased to a total of about 1% have been used in some ICs. However, the effect for improving the wire deformation is not satisfactory at the time of molding with the resin, leaving a problem of a decrease in the junction to the leads.
As means for achieving a high degree of strength, there have been proposed multi-layer wires of metals which are different between the core portion and the outer peripheral portion. For example, Japanese Unexamined Patent Publication (Kokai) No. 56-21354 teaches wires obtained by covering Ag cores with Au, and Japanese Unexamined Patent Publication (Kokai) No. 59-155161 teaches wires having cores of an electrically conducting metal of which the surfaces are plated with Au. Upon use, in combination the metals which are different between the cores and the outer peripheral portions, it is expected that the wires will satisfy the requirements of a large strength and a high junction property compared with the wires made of a single material employed by products in general. In practice, however, there has not been reported any use of multi-layer wires for the semiconductors.
To meet the need for high-density mounting in the future, therefore, the wires should not be simply those that satisfy their individual requirements and it is urged to develop a material which improves the overall properties.
The wires that suit the narrow-pitch junction in which the gap among the neighboring electrodes is not larger than 50 μm must simultaneously satisfy the wedge junction property and a leaning property, which are new problems, yet favorably improve high strength and high elasticity, loop controllability, junction property while suppressing the wire sweep that is a traditional problems.
Concerning the wires for realizing the narrow-pitch connection, study has been forwarded, using a gold alloy, using a material to substitute for gold and using multi-layer wires. Described below are problems that are encountered when the narrow-pitch connection is to be realized on a mass-production level by employing the above methods.
By utilizing the curing by solid solution, curing by precipitation and curing by the formation of a compound due to the addition of alloy elements to gold, and mutual action relative to the dislocation, it is made possible to increase the strength required for the conventional wires to some extent. Even by simply adding alloy elements, however, there is a limit to increasing the strength or to increasing the modulus of elasticity and, besides, it becomes difficult to suppress the deformation of wires when molded with the resin. When the amount of wire sweep is as large as 5% or more in the conventional molding technology, an increase in the strength of the wires is effective for suppressing the wire sweep because the wire deformation takes place chiefly in the plastic region. Due to the recent development in the resin molding technology, on the other hand, the wire sweep is becoming dominated by the elastic deformation, and an increase in the modulus of elasticity is becoming ever more important. To increase the modulus of elasticity of the gold alloy wires to be not smaller than 88 MPa, however, the action of only a solid solution of alloy elements and of precipitation is not enough. Further, if elements are added at a high concentration in order to increase the strength and the modulus of elasticity, new problems arise, such as oxidation of wire surfaces, occurrence of cavities when the balls are formed, drop in the junction properties of the ball portions, excess increase in the electric resistance, etc.
According to the traditional method of selecting the kind and concentration of elements that are added for utilizing the solid solution of alloy elements added to the wire material, precipitation thereof and formation of a compound, it is quite difficult to apply the gold bonding wires for ball junction to the very narrow-pitch connection on a mass production basis.
The above multi-layer wires constituted by the cores and the outer peripheral portions may have different properties, between the cores and the outer peripheries, and different latent properties can be expected. However, the production of the multi-layer wires is so complex that there remain problems that have to be solved for mass production, such as an increase in the cost of production due to an increase in the steps and new facilities, as well as very difficult quality control such as homogenization and stabilization of properties. In the multi-layer wires, particular properties can be improved relatively easily. However, complex wire properties required for the narrow-pitch connection have not still been totally satisfied leaving problems that must be solved in practical use.
Therefore, none of gold alloy materials that substitute for gold and multi-layer wires satisfy all of the properties required for the narrow-pitch connection. The bonding wires for narrow-pitch junction must have a large strength, a high elasticity and a high rigidity for suppressing the wire sweep and must, at the same time, satisfy the conflicting properties of improving the loop controllability and junction, while lowering the cost and minimizing an increase in the electric resistance.
As the pitch becomes smaller than 40 μm, further, there arise new problems such as leaning of the ball erecting portions, that, so far, were not regarded as problems. In a multi-pin/narrow-pitch connection, different wire lengths and different loop heights exist in the mounting of a single IC. Unlike the conventional mounting employing the same loop shape in the chip, however, this tends to develop a problem related to loop control. The problem which is drawing the most serious attention in recent years is that the wire-erecting portions fall near the ball junction permitting the neighboring wires to approach too close. This phenomenon is called leaning and is becoming a major cause for lowering the mass productivity of narrow-pitch connection.
The leaning of the ball-erecting portions cannot be easily improved despite the strength and modulus of elasticity of the wires being simply increased or, conversely, despite the strength being decreased. Even if the elongation after fracture frequently used in the mechanical properties related to the bonding wires is increased or decreased, the leaning is not suppressed. This is because the ball-erecting portions have already been deformed and distorted due to the heat at the time of melting the balls and due to the formation of loops and are dominated by properties different from those of mother wire. Therefore conventional method of improvement based on the mechanical properties faces a limit. To cope with the leaning, a wire material must be developed based upon a new concept.
Further, the wires of which the strength is simply increased are not capable of fully satisfying the deforming property, junction strength and long-term reliability at the wedge junction portions in the highly dense connection such as a narrow-pitch connection. No problem occurred in the wedge junction when the pitch was not shorter than 70 μm in the prior art. In narrow-pitch connections in the future, however, it will become important to improve the wedge junction property.
The factors that induce problems related to the wedge junction stem from fine wires, greatly decreased junction areas due to fine capillary ends and low junction temperatures as the narrow-pitch connection is effected chiefly for the board and the tape. That is, the problem related to the wedge junction has not so far been seriously tackled and must, hence, be handled as a new problem for realizing narrow pitches.
The wedge junction forms a special junction structure very different from the deformation of ball portions, such as wires undergoing complex behavior at high speeds receiving large deformation, good junction strength is maintained to meet the counterpart to which the wire is to be joined, and stabilizing the cutting shape of the wires in the step of forming small balls for a narrow pitch after the wedge is joined. So far, the wedge junction was maintained without the need of taking the above phenomena into consideration. Therefore, the factors related to the wire materials for improving the wedge junction have not been clarified, and practical examples of wire products enhancing the wedge junction properties have not been reported. Rather, it has often been pointed out that the highly strengthened wires generally exhibit lowered wedge junction properties at low temperatures. For example, if the wires are simply strengthened by the addition at high concentrations, the operation margin decreases for maintaining the wedge junction property. To meet the demand for narrowing the pitch, it becomes an important technical assignment to use strong and fine wires and to improve the wedge junction property.
In the wedge junction portion, further, it is necessary to improve the reliability in use in addition to the junction property. The silicon chip, metal frame, bonding wires and molding resin for covering them have different coefficients of thermal expansion and moduli of elasticity, and tend to develop heat distortion. It is considered that the thermal strain concentrates in the wedge junction portion resulting in breakage during the reflowing, during use where heat is generated, or through temperature cycles such as repeated cooling. A problem occurs concerning the thermal fatigue at the side of the wires as the wires become fine and the wedge junctions become fine. Accompanying the use of Pb-free solder in recent years, further, the reflowing temperature is becoming high, thus accelerating thermal fatigue. The factors related to the wire materials have not been clarified for improving reliability in the wedge junction. Reliability during use, such as fatigue resistance of the wedge junction portions having a greatly deformed complex structure, is greatly different from that of the wire deformation and junction at the time of controlling the loops or molding required so far for the wires. Therefore, this is beyond the conventional simple design of materials such as components and concentrations thereof.
It is therefore an object of the present invention to provide gold bonding wires for semiconductor elements having a large strength and a high flexural rigidity suited for narrowing the pitch, decreasing the wire size and lengthening the wire, having an improved junction property and suited for mass-production on an industrial scale, and a method of producing the wires.
From the above-mentioned point of view, the present inventors have engaged in study and development in an attempt to totally improve the strength, modulus of elasticity, wedge junction property and to suppress the flow of wires for realizing a narrow-pitch connection, as well as to improve the leaning of the ball-erecting portions, and have discovered, for the first time, that controlling the aggregate structure of the wires is important and effective.